export_of_non_library

The exported library '{0}' can't have a part-of directive.

The analyzer produces this diagnostic when an export directive references a part rather than a library.

Given a file part.dart containing

part of lib;

The following code produces this diagnostic because the file part.dart is a part, and only libraries can be exported:

library lib;
​
export 'part.dart';

Either remove the export directive, or change the URI to be the URI of the library containing the part.